replacement of the thermostat on 2003 Lincoln Aviator

My truck wasn't getting enought heat. It would blow out cold air while sitting still. And blow out warmer air while riding.My son put in a thermostat for me and now its running hot. It wasn't doing this before. Now i don't know what to do. Or is it something else that was suppose to be done after he put in this thermostat. I wish someone may have the answer for me what to do next. Thanks

He may have not bled the air out of the cooling system, and you may have an air pocket that is simulating an overheating condition as it tries to purge the air. Or, something may have gone wrong. Either way, the investment of your truck is too valuable to risk not checking it properly.